Corrosion behaviors of polymeric materials were classified into diffusion-controlled systems and reaction-controlled systems. In the former, in which the diffusion coefficient is small, the polymer decomposed from its surface. In the latter, in which the reaction rate of decomposition is small, the polymer degraded after penetration of the ambient solution. In the corrosion of polyamide (PA) by sulfuric acid, mechanism found to involve the simultaneous diffusion of sulfuric acid into PA and the hydrolysis of PA. A kinetic equation representing the decrease in molecular weight of PA by hydrolysis was then proposed, and the change in sulfuric acid concentration distribution over time was calculated using the experimentally determined diffusion coefficient of sulfuric acid into PA. Finally, the rate of PA corrosion was estimated numerically by combining the change in sulfuric acid concentration and the kinetic equation of molecular weight decrease. The analytical results agreed well with the experimental ones. A generalized corrosion model could be established by adjusting the diffusion coefficient and the rate constant of the kinetic equation in the analytical expression.